The treatment of a number of diseases including a broad range of anemias, hemoglobinopathies and malarial diseases will require a fundamental understanding of both cellular and molecular aspects of human erythropoiesis. Many experimental methodologies aimed at understanding this process are inherently limited by the use of nonhuman cells and cell lines derived from transformed cells. The prospective study of human erythropoiesis using peripheral blood derived cells has been limited by retrospective analyses associated with semisolid culture of individual cells and contaminating populations within bulk cultures. We have taken a direct approach toward the prospective study of the early and late transcriptional events that encompass human erythropoiesis by examining cells that proliferate in culture specifically in response to the hormone erythropoietin. Using flow cytometry to analyze liquid cultured blood from normal volunteers, we have identified and temporally phenotyped the erythroid continuum of cells present in bulk cultures. This approach permitted us to identify, quantitate and purify erythroblasts that are transcriptionally committed to erythroid differentiation at early (progenitor) and late (precursor) developmental stages. The progenitor cell population is pivotal for the study of proliferation and differentiation events associated with normal and abnormal human erythropoiesis. The precursor cell population is equally important for defining the transcriptional events required for terminal erythroid cell differentiation. Our goal is to fully characterize these cell populations using current molecular genetic methods in order to understand and manipulate their transcriptional patterns. We are generating a robust database describing the transcriptional profile of genes active in pure populations of erythroid progenitor and precursor cells that has been organized into an internet accessible resource (http://hembase.niddk.nih.gov/). Several thousand genes have been identified including several hundred novel transcripts. Individual genes within the database have been identified for further study on the basis of informatic, laboratory, and clinical screening assays and disease correlates. Directly related studies involving improvements in erythroid culture models, array based assays of genes transcribed in erythroid cells, flow cytometric assays of erythroid cells, and the design and production of therapeutic modalities aimed at the manipulation of gene expression in within the erythroid lineage are also being performed in the laboratory.